In a hard disk drive, servo data are used to position a magnetic head (hereinafter also referred to as a head) on a target track on a magnetic disk (hereinafter also referred to as a disk). Radial servo areas (servo regions) are provided on the disk, and each of the servo areas includes a plurality of servo tracks. The servo data, which include address and burst data, are written in each of the servo tracks.
The servo data are recorded on the disk through a servo writing process during manufacture of the hard disk drive. The servo writing process includes a self-servo writing process, which is carried out using the head of the hard disk drive after the disk and the head are assembled into the hard disk drive.
After writing one unit of servo data, the head moves a distance less than a track width of the servo track and writes the next unit of servo data so as to overwrite part of the servo data that have been written immediately previously. The remaining part of the servo data becomes a servo track. When the head moves inward on the disk, an inner track side of servo data is overwritten. When the head moves outward on the disk, an outer track side of each servo data is overwritten.
A skew angle (also called a yaw angle) formed by a tangent of a track and a line passing through centers of a write element of the head and a read element of the head is zero at a middle diameter track on the disk. The skew angle increases in a positive direction as the head moves outward, and increases in a negative direction as the head moves inward. If the skew angle is large, quality of the servo data may be degraded because the data are written by a side end of the write head. It is known that the degree of degradation differs according to whether the skew angle is positive or negative and which direction the head moves. That is, if the head writes servo data while moving inward, the quality tends to be degraded more when the skew angle is negative than when the skew angle is positive. When the head writes servo data while moving outward, the quality tends to be degraded more when the skew angle is positive than when the skew angle is negative.
To deal with this issue, a bidirectional writing method of writing servo data is proposed. In tracks located outside of a middle diameter track at which the skew angle is zero, the servo data are written as the head moves inward. In tracks located inside of the middle diameter track, the servo data are written as the head moves outward. The degradation of the servo data can be reduced by this bidirectional writing method.
Since the last track at the middle diameter in the bidirectional writing method is not overwritten with another track, a width of the last track is greater than the width of the other tracks, assuming that a width of normal track is smaller than the width of the write element. That is, the width of the last-written track is different from the width of the other tracks, which leads to discontinuity of track pitch. For that reason, the address and the burst data become irregular at the last track and servo control using such servo data may not be performed properly.